Method for performing electropolishing treatment on aluminum material

ABSTRACT

Provided is a method of performing electropolishing treatment on an aluminum material, which is capable of easily producing an aluminum material having an excellent outer appearance with luster and uniformity on an industrial scale. The method of performing electropolishing treatment on an aluminum material includes immersing the aluminum material in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material, the method including: performing, as pretreatment for the electropolishing treatment, anodic oxidation treatment; performing, during the electropolishing treatment, bubble-diffusion-preventing treatment; and performing, as post-treatment for the electropolishing treatment, film peeling treatment for an electropolishing film.

TECHNICAL FIELD

The present invention relates to a method of performing electropolishing treatment on an aluminum material formed of aluminum or an aluminum alloy, and more particularly, to a method of performing electropolishing treatment on an aluminum material, which is suitable for producing an aluminum material having an excellent outer appearance with luster and uniformity.

BACKGROUND ART

In electropolishing treatment of an aluminum material, a large amount of hydrogen gas is generated along with an electrolytic reaction of the aluminum material during the electropolishing treatment, and the hydrogen gas is formed into bubbles in an electrolytic solution to adhere to the surface of the aluminum material serving as a material to be treated. The bubbles adhering to the surface inhibit the electrolytic reaction on the surface of the aluminum material. As a result, portions to which the bubbles adhere become apparent as point-like defects after the electropolishing and impair luster, uniformity, and the like on the surface of the aluminum material after the electropolishing.

In view of the foregoing, in order to obtain an aluminum material having an outer appearance excellent in luster and uniformity, it is necessary to prevent the bubbles from adhering to the surface of the aluminum material serving as the material to be treated during the electropolishing treatment or to remove the adhering bubbles. For example, there have been proposed an oscillation method for oscillating an aluminum material (Non Patent Literature 1), an electrolytic solution stirring method for stirring an electrolytic solution by shaking of an aluminum material or with an oscillating vane stirrer (Non Patent Literature 2), and the like.

However, in the oscillation method and the electrolytic solution stirring method disclosed in Non Patent Literatures 1 and 2, when the aluminum material has a complicated shape or a large area, it is difficult to completely prevent the adhesion of bubbles uniformly over the entire surface of the aluminum material and to completely remove the adhering bubbles. Further, the electropolishing treatment is performed through use of a concentrated acid, and hence it is necessary to manufacture a device configured to oscillate or shake the aluminum material or the oscillating vane stirrer with an expensive material excellent in acid resistance. Thus, considerable labor and cost are required also for maintaining and managing a facility. The methods are not preferred from the viewpoint of industry.

Further, when the electropolishing treatment is finished and current application is stopped, non-uniform dissolution occurs on the surface of the aluminum material after the electropolishing in an electropolishing solution having a strong dissolution ability, and the outer appearance and a mirror finishing property of the surface are impaired. Therefore, an anodic oxide film generated during the electropolishing treatment (hereinafter referred to as “electropolishing film”) is sometimes caused to remain intentionally. However, depending on the application, it may be necessary to remove the electropolishing film remaining on the surface of the aluminum material after the electropolishing.

Therefore, the electropolishing film remaining on the surface of the aluminum material after the electropolishing has hitherto been removed, and for example, film peeling treatment involving immersing the aluminum material after the electropolishing in a peeling solution, such as a sodium hydroxide aqueous solution, a mixed aqueous solution of phosphoric acid and chromic acid, or an aqueous solution of sulfuric acid or nitric acid and a fluorine compound, to thereby dissolve and remove the anodic oxide film remaining on the surface of the aluminum material is performed (Non Patent Literature 3).

The film peeling treatment method involving using a peeling solution formed of a mixed aqueous solution of phosphoric acid and chromic acid or an aqueous solution of sulfuric acid or nitric acid and a fluorine compound is an excellent method because the electropolishing film can be selectively dissolved and removed. However, chromic acid and the fluorine compound that are main components are toxic substances from the viewpoint of environment sanitation, and currently, it is actually difficult to use these substances. Further, in the film peeling treatment method involving using a peeling solution formed of a sodium hydroxide aqueous solution, the sodium hydroxide aqueous solution dissolves aluminum or an aluminum alloy as a base material. Therefore, the base material is also inevitably dissolved along with the dissolution of the electropolishing film on the surface, and as a result, luster exhibited by the electropolishing treatment is traded off to some degree.

CITATION LIST Non Patent Literature

-   [NPL 1] “Aluminum surface technology handbook,” p. 106, Oct. 17,     1980, issued by Light Metal Publishing Co., Ltd. -   [NPL 2] Aluminum finishing society of Kinki, No. 188, 1997, pp. 6-10 -   [NPL 3] “Aluminum Encyclopedia”, p. 1017, Nov. 1, 1969, issued by     Light Metal Syndicate

SUMMARY OF INVENTION Technical Problem

In view of the foregoing, the inventors of the present invention have made extensive investigations regarding a method of performing electropolishing treatment on an aluminum material, which solves various problems caused by the electropolishing treatment of an aluminum material described above and which can easily produce an aluminum material having an excellent outer appearance with luster and uniformity on an industrial scale. The inventors of the present invention have also made the following investigations regarding a problem of point-like defects caused by the adhesion of bubbles generated along with chemical dissolution of the aluminum material at the start of and during the electropolishing treatment of the aluminum material and a problem involved in dissolution removal of an electropolishing film remaining on the surface of the aluminum material performed after the electropolishing of the aluminum material. As a result, the inventors of the present invention have developed the following method capable of solving the problems.

First, the inventors of the present invention have made the following investigations and development regarding the problem of point-like defects caused by the adhesion of bubbles at the start of and during the electropolishing treatment of the aluminum material.

Specifically, the inventors of the present invention have made detailed investigations on a generation mechanism of the point-like defects generated during the electropolishing treatment, and as a result, have found that the point-like defects generated on the surface of the aluminum material after the electropolishing are not pits (dents) as considered hitherto but protrusions. Further, the inventors of the present invention have found the following facts. Hydrogen gas that causes such point-like defects is not merely generated as a result of an electrolytic reaction during the electropolishing treatment under application of a current. When the aluminum material is immersed in an electrolytic treatment solution under no application of a current before the electropolishing treatment, the aluminum material is brought into contact with the electrolytic treatment solution to cause a chemical dissolution reaction. Hydrogen gas generated through the reaction is formed into bubbles to adhere to the surface of the aluminum material, and the adhering bubbles also inhibit the electrolytic reaction during the electropolishing treatment performed later, to thereby cause the point-like defects.

Therefore, in the related-art oscillation method for oscillating an aluminum material serving as a material to be treated and the related-art electrolytic solution stirring method involving using the shaking of an aluminum material serving as a material to be treated or an oscillating vane stirrer, as compared to the case where the aluminum material is left to stand still without taking any measure, the problem of the point-like defects caused by the adhesion of bubbles during the electropolishing treatment of the aluminum material can be solved significantly. However, in the oscillation method and the electrolytic solution stirring method, a great deal of experience and a high degree of skill are required for adjusting the strength and magnitude of the oscillation, shaking, or stirring. Further, the inventors of the present invention have found the following facts. The adhesion of bubbles generated when the aluminum material is immersed in the electrolytic treatment solution under no application of a current at the beginning of the electropolishing treatment cannot be prevented. Further, it is difficult to completely remove the bubbles that have once adhered to the surface of the aluminum material by the oscillation method or the electrolytic solution stirring method during the electropolishing treatment. Further, the related-art methods are not applicable to a high-quality and excellent outer appearance with luster and uniformity, particularly as a mirror surface in which even slight defects are not accepted with respect to the surface of the aluminum material after the electropolishing.

In view of the foregoing, the inventors of the present invention have focused attention on the following facts. During the electropolishing treatment of the aluminum material, in the electrolytic treatment solution having an extremely strong ability to dissolve the aluminum material, any one of an electrolytic reaction in which an electropolishing film is formed and a dissolution reaction in which metal aluminum is chemically dissolved in the electrolytic treatment solution occurs on the surface of the aluminum material. In an initial stage of the electropolishing treatment, in order to perform the electrolytic reaction in the electrolytic treatment solution having a strong dissolution ability, it is necessary to cause a large current to flow temporarily so that the electropolishing film is formed rapidly while overcoming the chemical dissolution of the metal aluminum. As a result, the inventors of the present invention have developed a method including performing, as pretreatment for the electropolishing treatment, anodic oxidation treatment under predetermined conditions so as to suppress the generation of hydrogen gas caused by the chemical dissolution of the surface of the aluminum material in the initial stage of the electropolishing treatment by an anodic oxide film formed in advance on the surface of the aluminum material, and utilizing the anodic oxide film as an electropolishing film required in the initial stage of the electropolishing treatment. Thus, the present invention has been completed.

Further, the inventors of the present invention have made the following investigations and development regarding the problem of the point-like defects caused by the adhesion of bubbles at the start of and during the electropolishing treatment of the aluminum material.

Specifically, the inventors of the present invention have made further investigations regarding how the generation and adhesion of bubbles occur during the electropolishing treatment of the aluminum material. As a result, the inventors of the present invention have found that, during the electropolishing treatment after the start of application of a current, the generation of the hydrogen gas hardly occurs on an aluminum material side serving as an anode, and the generation of the hydrogen gas occurs mainly on a cathode side serving as a counter electrode of the aluminum material. Further, the inventors of the present invention have found that the bubbles of the hydrogen gas generated on the cathode side diffuse to the aluminum material side serving as the anode, and the diffusing bubbles adhere to the surface of the aluminum material, to thereby cause the problem of the point-like defects.

In view of the foregoing, the inventors of the present invention have made various investigations regarding a method of preventing the bubbles of the hydrogen gas generated on the cathode side from diffusing to the aluminum material side serving as the anode. As a result, the inventors of the present invention have found that the bubbles of the hydrogen gas generated on the cathode can be effectively prevented from diffusing to the aluminum material side serving as the anode by forming a cathode partition chamber formed of a liquid-permeable material which is non-permeable to the bubbles to be generated in an electrolytic treatment layer (non-permeable to bubbles) and is permeable to the electrolytic treatment solution so that conductivity between the electrodes can be ensured, and arranging the cathode in the cathode partition chamber. Thus, the present invention has been completed.

Further, the inventors of the present invention have made the following investigations and development regarding the problem involved in dissolution removal of the electropolishing film remaining on the surface of the aluminum material performed after the electropolishing of the aluminum material.

Specifically, in the electropolishing treatment of the aluminum material, when the aluminum material is kept immersed in the electrolytic treatment solution after the completion of the treatment, the surface of the aluminum material is chemically dissolved with the electrolytic treatment solution, and non-uniform chemical dissolution occurs at this time to form local convex portions, which cause the point-like defects. Therefore, it is necessary to remove the aluminum material after the electropolishing from the electrolytic treatment solution immediately after the completion of the treatment. However, the electropolishing treatment is a method in which a porous electropolishing film is dissolved at high speed while the porous electropolishing film is formed, and hence the electrolytic treatment solution has a strong ability to dissolve the aluminum material, and the chemical dissolution of the surface of the aluminum material starts immediately after application of a current is stopped. Therefore, it is difficult to prevent non-uniform chemical dissolution immediately after the completion of the electropolishing treatment while achieving an excellent mirror finishing property by keeping the electrolytic reaction and the chemical dissolution reaction during the electropolishing treatment in a satisfactory state.

In view of the foregoing, as a method of solving the problem caused by the chemical dissolution after the electropolishing, the electropolishing film is caused to remain intentionally on the surface of the aluminum material after the electropolishing, and the electropolishing film remaining on the surface is dissolved and removed after the electropolishing. However, as described above, the dissolution removal of the electropolishing film to be performed after the electropolishing has a problem.

In view of the foregoing, the inventors of the present invention have made further investigations regarding the dissolution removal of the electropolishing film on the surface of the aluminum material after the electropolishing, and have focused attention on the following facts. The dissolution of the electropolishing film (oxide) is a chemical dissolution reaction not involving the movement of a charge. In contrast, the dissolution of a base material (metal) is an electrochemical reaction involving the movement of a charge. Then, the inventors of the present invention have made investigations so as to develop a post-treatment solution excellent in such selective dissolution ability as to accelerate the chemical dissolution reaction while suppressing the electrochemical reaction, and have attained a sulfuric acid solution having a pH of 2 or less and containing sulfuric acid and an amine. Thus, the present invention has been completed.

Thus, an object of the present invention is to provide a method of performing electropolishing treatment on an aluminum material, which can easily produce an aluminum material having an excellent outer appearance with luster and uniformity on an industrial scale.

Specifically, the present application provides the following first to fourth inventions, and the first to fourth inventions are sometimes referred to as “the present invention” collectively.

Solution to Problem

According to a first invention of the present application, there is provided a method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material,

the method including, as pretreatment for the electropolishing treatment, performing anodic oxidation treatment including subjecting the aluminum material to anodic oxidation in a pretreatment tank arranged separately from the electrolytic treatment tank at a voltage lower than the electrolysis voltage during the electropolishing treatment through use of a pretreatment solution formed of a polybasic acid aqueous solution having a weak ability to dissolve the aluminum material as compared to the electrolytic treatment solution, to thereby form an anodic oxide film on the surface of the aluminum material.

Further, according to a second invention of the present application, there is provided a method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material,

the electrolytic treatment tank including a cathode partition chamber which is formed of a material non-permeable to bubbles of hydrogen gas to be generated on a cathode serving as a counter electrode of the aluminum material and permeable to a liquid, is configured to partition the cathode from the aluminum material, and includes a discharge port for discharging the hydrogen gas out of the electrolytic treatment tank,

the method including, during the electropolishing treatment of the aluminum material, performing bubble-diffusion-preventing treatment including: preventing the bubbles of the hydrogen gas generated on the cathode in the cathode partition chamber from diffusing to an aluminum material side; and discharging the hydrogen gas out of the electrolytic treatment tank through the discharge port.

Further, according to a third invention of the present application, there is provided a method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material,

the method including, as post-treatment for the electropolishing treatment, performing film peeling treatment including immersing the aluminum material after the electropolishing treatment in a post-treatment solution formed of a sulfuric acid solution having a pH of 2 or less and containing sulfuric acid and an amine, to thereby dissolve and remove an electropolishing film present on the surface of the aluminum material.

Further, according to a fourth invention of the present application, there is provided a method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material,

the method including performing two kinds or three kinds of the treatments selected from the anodic oxidation treatment as the pretreatment of the first invention, the bubble-diffusion-preventing treatment during the electropolishing treatment of the second invention, and the film peeling treatment of the electropolishing film as the post-treatment of the third invention.

In the present invention, there is no particular limitation on the aluminum material formed of aluminum or an aluminum alloy to be subjected to the electropolishing treatment, and various aluminum materials required to be mirror-finished by the electropolishing treatment are applicable. Examples of the aluminum material include Al—Cu-based 2000-based materials, Al—Mg-based 5000-based materials, and Al—Mg—Si-based 6000-based materials. Of those, in particular, as an aluminum material having a high aluminum purity (Al purity) and required to be highly mirror-finished, there may be given, for example, a high-purity aluminum material having a purity of 99.99% or more, a pure aluminum-based 1000-based material (such as A1050 material), and the like.

Further, in the aluminum material, the surface thereof may be subjected to preliminary mirror-finishing treatment in advance by means of buffing, cutting work, or chemical polishing, and the present invention is also effective for an aluminum material having a surface subjected to such preliminary mirror-surface finishing in advance.

Further, in the present invention, the treatment conditions for the electropolishing treatment of the aluminum material in the electrolytic treatment solution in the electrolytic treatment tank do not particularly differ from the conditions of related-art electropolishing treatment of an aluminum material, and treatment conditions for the related-art electropolishing treatment can be directly adopted with regard to the composition of the electrolytic treatment solution, the electrolysis voltage, the treatment temperature, the treatment time, the inrush current at the start of the electropolishing treatment, and other conditions. Herein, as the electrolytic treatment solution, there may be given, for example, a solution having a composition of phosphoric acid and sulfuric acid (volume ratio: 7:3), a composition of phosphoric acid and sulfuric acid (volume ratio: 1:1), and the like.

The first invention of the present application is the method of performing electropolishing treatment on an aluminum material, the method including, as pretreatment for the electropolishing treatment, performing anodic oxidation treatment including subjecting the aluminum material to anodic oxidation in a pretreatment tank arranged separately from the electrolytic treatment tank to be used in the electropolishing treatment at a voltage lower than the electrolysis voltage during the electropolishing treatment through use of a pretreatment solution formed of a polybasic acid aqueous solution having a weak ability to dissolve the aluminum material as compared to the electrolytic treatment solution, to thereby form an anodic oxide film on the surface of the aluminum material.

In this case, it is important in the anodic oxidation treatment as the pretreatment to perform the anodic oxidation treatment in the pretreatment tank arranged separately from the electrolytic treatment tank. Further, it is important in the anodic oxidation treatment to subject the aluminum material to the anodic oxidation at a voltage lower than the electrolysis voltage during the electropolishing treatment through use of the pretreatment solution formed of a polybasic acid aqueous solution having a weak ability to dissolve the aluminum material as compared to the electrolytic treatment solution during the electropolishing treatment. With this, an intended anodic oxide film can be formed easily on the surface of the aluminum material during the pretreatment. Further, through adoption of such anodic oxidation treatment as the pretreatment, a power source specification during the electropolishing treatment can be directly utilized, and hence it is not necessary to separately prepare a dedicated power source. Further, it is not necessary to form the anodic oxide film on the surface of the aluminum material by causing a large current to flow in an initial stage of the electropolishing treatment, and hence the capacity of the power source during the electropolishing treatment can be reduced, with the result that a relatively small and inexpensive power source device can be adopted.

Further, as the pretreatment solution formed of a polybasic acid aqueous solution having a weak ability to dissolve the aluminum material as compared to the electrolytic treatment solution during the electropolishing treatment to be used in the anodic oxidation treatment as the pretreatment, specifically, there are given, for example, a sulfuric acid aqueous solution having a composition of a sulfuric acid concentration of 15 mass %, an oxalic acid aqueous solution having a composition of an oxalic acid concentration of 2 mass %, and the like. When the electrolytic treatment solution and the pretreatment solution use the same acid, for example, when the acid is sulfuric acid, a sulfuric acid aqueous solution having a sulfuric acid concentration lower than that of the electrolytic treatment solution or having a pH value higher than that of the electrolytic treatment solution is used as the pretreatment solution. Further, the treatment conditions for the anodic oxidation treatment as the pretreatment do not differ from the treatment conditions for ordinary anodic oxidation treatment.

Further, in the anodic oxidation treatment as the pretreatment, the thickness of the anodic oxide film to be formed on the surface of the aluminum material maybe the thickness formed by general anodic oxidation treatment. There is no particular limitation on the thickness, and the thickness is generally from about several tens of nanometers to about several tens of micrometers.

Further, the second invention of the present application is the method of performing electropolishing treatment on an aluminum material, the electrolytic treatment tank including a cathode partition chamber which is formed of a material non-permeable to bubbles of hydrogen gas to be generated on a cathode serving as a counter electrode of the aluminum material and permeable to a liquid, is configured to partition the cathode from the aluminum material, and includes a discharge port for discharging the hydrogen gas out of the electrolytic treatment tank,

the method including, during the electropolishing treatment of the aluminum material, performing bubble-diffusion-preventing treatment including: preventing the bubbles of the hydrogen gas generated on the cathode in the cathode partition chamber from diffusing to an aluminum material side; and discharging the hydrogen gas out of the electrolytic treatment tank through the discharge port.

In this case, it is sufficient that the cathode partition chamber configured to partition the cathode from the aluminum material serving as the anode be formed of at least a material non-permeable to bubbles and permeable to a liquid, include a partition wall for partitioning the cathode and the aluminum material and the discharge port for the hydrogen gas, and be able to discharge the hydrogen gas generated in the cathode partition chamber out of the tank through the discharge port. Further, as a material for forming the cathode partition chamber, there may be given, for example, a porous film made of Teflon (trademark), a glass sintered filter, and a filter medium formed of glass, such as glass fibers.

Further, the third invention of the present application is the method of performing electropolishing treatment on an aluminum material, the method including, as post-treatment for the electropolishing treatment, performing film peeling treatment including immersing the aluminum material after the electropolishing treatment in a post-treatment solution formed of a sulfuric acid solution having a pH of 2 or less and containing sulfuric acid and an amine, to thereby dissolve and remove an electropolishing film present on the surface of the aluminum material.

As the post-treatment solution to be used in the film peeling treatment as the post-treatment, the sulfuric acid solution having a pH of 2 or less and containing sulfuric acid and an amine is used. It is desired that the sulfuric acid solution contain at least sulfate ions and/or sulfite ions and ammonium ions derived from the amine in the aqueous solution, and the pH of the sulfuric acid solution be generally 2 or less, preferably 1.5 or less. When the pH value of the sulfuric acid solution is more than 2, there is a risk in that the dissolution of the film may hardly proceed. Such post-treatment solution is desirably a sulfuric acid solution obtained by dissolving a sulfate of an amine in water, a sulfuric acid solution obtained by dissolving sulfuric acid and an amine in water, a sulfuric acid solution obtained by dissolving sulfuric acid, an amine, and a sulfate of an amine in water, or a sulfuric acidic solution obtained by dissolving amidosulfuric acid (sulfamic acid) or an amidosulfuric acid salt thereof, such as an ammonium salt thereof, in water, more preferably an aqueous solution of amidosulfuric acid. Further, examples of the amine for preparing the sulfuric acid solution as the post-treatment solution may include ammonia, and an alkylamine, such as methylamine or propylamine.

As a specific example of the film peeling treatment performed in the third invention, for example, in the case of using a 3 mass % sulfamic acid aqueous solution as the post-treatment solution, there is given a method involving immersing the aluminum material in the solution under the treatment conditions of 70° C. and 10 minutes.

Further, the fourth invention of the present application is the method of performing electropolishing treatment on an aluminum material, the method including performing two kinds or three kinds of the treatments selected from the anodic oxidation treatment as the pretreatment of the first invention, the bubble-diffusion-preventing treatment during the electropolishing treatment of the second invention, and the film peeling treatment of the electropolishing film as the post-treatment of the third invention.

In the fourth invention, when two kinds of the treatments are combined, the treatments of the first invention, the second invention, and the third invention described above may be combined arbitrarily. In the electropolishing treatment of the aluminum material, for example, the anodic oxidation treatment of the first invention and the bubble-diffusion-preventing treatment of the second invention may be combined. Further, the anodic oxidation treatment of the first invention and the coating peeing treatment of the third invention may be combined. Further, the bubble-diffusion-preventing treatment of the second invention and the film peeling treatment of the third invention may be combined. Further, when three kinds of the treatments are combined, the anodic oxidation treatment of the first invention, the bubble-diffusion-preventing treatment of the second invention, and the film peeling treatment of the third invention are combined to perform the electropolishing treatment of the aluminum material.

When those two kinds or three kinds of the treatments are combined to perform the electropolishing treatment of the aluminum material, the effects obtained in each treatment can be attained in each step of the pretreatment, the electropolishing treatment, and the post-treatment. Therefore, it is desired that those two kinds or three kinds of the treatments be performed in combination in accordance with the requirements for the outer appearance, such as luster and uniformity, required for the aluminum material to be subjected to the electropolishing treatment.

Advantageous Effects of Invention

According to the first invention, when the electropolishing treatment of the aluminum material is performed, particularly, when the aluminum material is immersed in the electrolytic treatment solution in the electrolytic treatment tank, the generation of the hydrogen gas caused by the chemical dissolution reaction on the surface of the aluminum material can be suppressed, the point-like defects caused by the adhesion of bubbles can be reduced significantly, and the outer appearance excellent in luster and uniformity can be achieved in the aluminum material after the electropolishing. Further, it is not necessary to cause a large current to flow in an initial stage of the electropolishing treatment, and thus a power source facility can also be downsized.

Further, according to the second invention, the hydrogen gas to be generated on the cathode serving as a counter electrode of the aluminum material does not diffuse to form bubbles on the surface of the aluminum material. Therefore, the point-like defects caused by the adhesion of the bubbles can be prevented, and the outer appearance excellent in luster and uniformity can be achieved in the aluminum material after the electropolishing.

Further, according to the third invention, the electropolishing film remaining on the aluminum material after the electropolishing can be selectively dissolved and removed by the film peeling treatment, and the outer appearance excellent in luster and uniformity can be achieved in the aluminum material after the electropolishing.

Still further, according to the fourth invention, through combination of the treatments of the first invention, the second invention, and the third invention described above, the outer appearance excellent in luster and uniformity can be achieved in the aluminum material after the electropolishing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 are each an explanatory view for illustrating a concept of the first invention. FIG. 1(a) is an illustration of an anodic oxidation treatment step as pretreatment, and FIG. 1(b) is an illustration of an electropolishing treatment step.

FIG. 2 are each an explanatory view for illustrating a concept of the second invention. FIG. 2(A) is an illustration of a related-art electropolishing treatment step, and FIG. 2(B) is an illustration of an electropolishing treatment step of the second invention.

DESCRIPTION OF EMBODIMENTS

Now, an embodiment of a first invention is described with reference to the conceptual views of the invention illustrated in FIGS. 1. Further, an embodiment of a second invention is described with reference to the conceptual views of the invention illustrated in FIGS. 2. Further, a preferred embodiment of a third invention is described.

FIG. 1 are each an explanatory view for illustrating a concept of a method of performing electropolishing treatment on an aluminum material according to the first invention, and a treatment process including an anodic oxidation treatment step (a) as pretreatment and an electropolishing treatment step (b) is illustrated.

In FIG. 1, in the anodic oxidation treatment step (a), a pretreatment solution 3 for anodic oxidation treatment to be performed as pretreatment is filled into a pretreatment tank 2 arranged separately from an electrolytic treatment tank 1 in the electropolishing treatment step (b). An aluminum material 4 serving as a material to be treated is set as an anode, and a DC voltage is applied between the anode and a cathode 5 serving as a counter electrode of the aluminum material 4 to form an anodic oxide film 6 on the surface of the aluminum material.

Next, in the electropolishing treatment step (b) of FIG. 1, an electrolytic treatment solution 7 is filled into the electrolytic treatment tank 1, and then, the aluminum material 4 serving as the material to be treated, in which the anodic oxide film 6 has been formed on the surface in the anodic oxidation treatment step (a) as the pretreatment, is immersed in the electrolytic treatment solution 7. After that, the aluminum material 4 is set as an anode, and a DC voltage is applied between the anode and a cathode 8 serving as a counter electrode of the aluminum material 4 to perform electropolishing treatment on the surface of the aluminum material 4.

In the electropolishing treatment step (b), at the time of immersion (S1) of the aluminum material 4 in the electrolytic treatment solution 7, the aluminum material 4 serving as the material to be treated is immersed in the electrolytic treatment solution 7 under no application of a current. However, the anodic oxide film 6 has been formed on the surface of the aluminum material 4, and hence a metal surface of the aluminum material 4 is not brought into direct contact with the electrolytic treatment solution 7 at the time of the immersion (S1) under no application of a current. Further, the anodic oxide film 6 on the surface of the aluminum material 4, which is brought into direct contact with the electrolytic treatment solution 7, is gradually dissolved into the electrolytic treatment solution 7, and hydrogen gas is not generated at this time.

Further, in the electropolishing treatment step (b), when a DC voltage is applied between the aluminum material 4 serving as the anode and the cathode 8 to start the electropolishing treatment, the anodic oxide film 6 on the surface of the aluminum material 4 is gradually dissolved into the electrolytic treatment solution 7 to be eliminated at the time of electrolytic treatment (S2) of the aluminum material 4. Further, the generation of an electropolishing film 9 and the chemical dissolution of metal aluminum occur on the surface of the aluminum material 4, and as a result, the anodic oxide film 6 is gradually dissolved to be eliminated. Further, the surface of the aluminum material 4 is subjected to electropolishing.

After the electropolishing treatment is completed as described above, the application of a current is stopped, and the aluminum material 4 after the electropolishing is immediately pulled up from the electrolytic treatment tank 1. Then, the aluminum material 4 is washed with pure water and dried with air to provide the aluminum material after the electropolishing.

In this case, as illustrated in FIG. 1, when the electropolishing film 9 is caused to remain intentionally on the surface of the aluminum material 4 after the electropolishing at the time of completion of the application of a current (S3) in the electropolishing treatment, and thereafter, the electropolishing film 9 remaining on the surface is removed, the third invention is performed successively. The third invention described below is a method effective for selectively dissolving and removing the electropolishing film on the surface of the aluminum material generated by the electropolishing treatment. Therefore, the third invention does not need to be performed successively after the first invention, and needless to say, may be performed successively after related-art electropolishing treatment.

In the third invention, coating peeing treatment including immersing the aluminum material after the electropolishing, which has been pulled up from the electropolishing solution at the time of completion of the electropolishing treatment, in a post-treatment solution formed of a sulfuric acid solution having a pH of 2 or less and containing sulfuric acid and an amine, to thereby selectively dissolve and remove the electropolishing film remaining on the surface of the aluminum material after the electropolishing.

After the film peeling treatment is completed, the aluminum material after the electropolishing is pulled up from a post-treatment tank. Then, the aluminum material is immediately washed with pure water and dried with air to provide the aluminum material after the electropolishing as a product.

Next, FIG. 2 are each an explanatory view for illustrating a concept of a method of performing electropolishing treatment on an aluminum material according to the second invention. FIG. 2(A) is an explanatory view for illustrating a related-art electropolishing treatment step using an electrolytic treatment tank, and further, FIG. 2B is an explanatory view for illustrating an electropolishing treatment step using an electrolytic treatment tank according to the second invention.

In FIG. 2(A) in which the electrolytic treatment tank 1 of the related-art is illustrated, the electrolytic treatment solution 7 is filled into the electrolytic treatment tank 1, and the aluminum material 4 serving as the material to be treated is immersed in the electrolytic treatment solution 7. After that, the aluminum material 4 is set as an anode, and a DC voltage is applied between the anode and the cathode 8 serving as a counter electrode of the aluminum material 4 to perform the electropolishing treatment on the surface of the aluminum material 4. Then, in the related-art electropolishing treatment using the electrolytic treatment tank 1, during the electropolishing treatment, bubbles 10 of hydrogen gas are generated on the cathode 8 side and diffuse into the electrolytic treatment solution 7. Most of the bubbles 10 fly into the atmosphere from a liquid surface of the electrolytic treatment solution 7. However, part of the bubbles 10 diffuses to the aluminum material 4 side serving as the anode to adhere to the surface of the aluminum material 4, which causes the generation of point-like defects on the surface of the aluminum material after the electropolishing.

In contrast, in the second invention, as illustrated in FIG. 2B, a cathode partition chamber 11 is formed in the electrolytic treatment tank 1. The cathode partition chamber 11 is formed of a material which is non-permeable to the bubbles 10 of the hydrogen gas generated on the cathode 8 serving as the counter electrode of the aluminum material 4 and is permeable to a liquid. The cathode partition chamber 11 is configured to partition the cathode 8 from the aluminum material 4 and includes a discharge port 12 for discharging the generated bubbles 10 out of the tank. During the electropolishing treatment of the aluminum material 4, the bubbles 10 generated on the cathode 8 in the cathode partition chamber 11 are prevented from diffusing to the aluminum material 4 side and are discharged out of the tank through the discharge port 12. By virtue of the bubble-diffusion-preventing treatment, the bubbles 10 of the hydrogen gas generated on the cathode 8 during the electropolishing treatment can be prevented from adhering to the surface of the aluminum material 4.

EXAMPLES

Now, a method of performing electropolishing treatment on an aluminum material of the present invention is described on the basis of Examples and Comparative Examples.

Examples 1 to 3

A plate material having an Al purity of 99.99 mass % was used as an aluminum material, and an aluminum piece having dimensions of 50 mm×50 mm×10 mm was cut out from the plate material. The aluminum piece was subjected to anodic oxidation treatment as pretreatment under treatment conditions (voltage, electrical quantity, and temperature) shown in Table 1 through use of a pretreatment solution shown in Table 1. The resultant aluminum piece was washed with water and dried to provide a pretreated aluminum piece of each of Examples 1 to 3.

The pretreated aluminum piece of each of Examples and Comparative Example thus obtained was subjected to electropolishing treatment under treatment conditions (temperature, voltage, time, and inrush current) shown in Table 1 through use of an electrolytic treatment solution shown in Table 1. The resultant aluminum piece was immediately washed with water and dried to provide an aluminum piece (test piece) after the electropolishing of each of Examples 1 to 3.

Comparative Example 1

The same aluminum piece as that used in each of Examples 1 to 3 described above was used to be subjected to electropolishing treatment under treatment conditions (temperature, voltage, time, and inrush current) shown in Table 1 through use of an electrolytic treatment solution shown in Table 1 without being subjected to anodic oxidation treatment as pretreatment. The resultant aluminum piece was immediately washed with water and dried to provide an aluminum piece (test piece) after the electropolishing of Comparative Example 1.

[Evaluation of Bubble Adhesion-Suppressing Property During Treatment]

During the electropolishing treatment in Examples 1 to 3 and Comparative Example 1 described above, the surface of the aluminum piece was visually observed from outside of a treatment tank made of glass, and bubbles of hydrogen gas adhering to the surface of the aluminum piece of each of Examples and Comparative Example were examined. Then, a suppression effect on bubble adhesion during the electropolishing treatment (bubble adhesion-suppressing property during treatment) was evaluated based on the following criteria: ◯: no adhesion of bubbles; Δ: adhesion of two or less bubbles (per 1 cm²); and x: adhesion of three or more bubbles (per 1 cm²).

The results are shown in Table 1.

[Evaluation of Outer Appearance Observation]

Further, each of the test pieces obtained in Examples 1 to 3 and Comparative Example 1 described above was visually observed for the following outer appearance, and examined and evaluated for mirror luster, point-like defects, and presence or absence of interference color.

Regarding the mirror luster, the degree of bubble adhesion during the electropolishing treatment was evaluated by visually observing how an object is reflected on a sample based on the following criteria: ◯: the object is reflected on the sample clearly without being distorted; Δ: the object is reflected on the sample clearly but is partially distorted; and x: the object is distorted significantly and is not reflected on the sample.

Further, regarding the point-like defects, the surface of each of the test pieces was visually observed under a fluorescent lamp, and the number of visually recognized point-like defects was counted and evaluated based on the following criteria: ◯: 0 pieces/cm²; Δ: 1 piece/cm² or more and less than 3 pieces/cm³; and x: 3 pieces/cm² or more.

Regarding the presence or absence of interference color, each of the test pieces was tilted by 70° under a fluorescent lamp, and the surface thereof was visually observed. Then, interference color caused by an electropolishing film was confirmed and evaluated based on the following criteria: ◯: no interference color is seen; Δ: interference color is seen in part of the sample; and x: an interference pattern is seen over the entire sample.

The results are shown in Table 1.

TABLE 1 Example Comparative 1 2 3 Example 1 Pretreatment Pretreatment 15 mass % 2 mass % oxalic 5 mass % tartaric — (anodic oxidation solution sulfuric acid acid acid treatment) Temperature (° C.) 20 20 20 — Voltage (V) 15 30 30 — Electrical 0.1 0.1 0.1 — quantity (C/cm²) Electropolishing Electrolytic Sulfuric Sulfuric Sulfuric Sulfuric treatment treatment acid-phosphoric acid-phosphoric acid-phosphoric acid-phosphoric solution acid acid acid acid (mass ratio) (1:5) (1:5) (1:5) (1:5) Temperature (° C.) 60 60 60 60 Voltage (V) 30 30 30 30 Time (min) 5 5 5 5 Inrush current 20 15 15 30 (A/dm²) Bubble adhesion-suppressing property Δ Δ Δ x during treatment Outer appearance Point-like defect 1 1 1 8 observation (piece(s)/cm²) evaluation Mirror luster ∘ ∘ ∘ x

Examples 4 and 5

In a tank for electropolishing treatment, a cathode partition chamber which was configured to partition a cathode from an anode (aluminum material) and included a discharge port for discharging bubbles of hydrogen gas to be generated during the electropolishing treatment out of the window was formed through use of a porous filter made of Teflon (trademark) (Example 4) or a filter made of glass fibers (Example 5), Teflon and the glass fibers being each a material non-permeable to bubbles of hydrogen gas and permeable to a liquid, to thereby constitute an electrolytic treatment tank.

Further, as the aluminum material, the same aluminum piece having an Al purity of 99.99 mass % as that used in each of Examples 1 to 3 described above was used.

Next, an electrolytic treatment solution (solution containing sulfuric acid and phosphoric acid in a mass ratio of 1:5) shown in Table 2 was loaded into the electrolytic treatment tank, and the aluminum piece was subjected to electropolishing treatment while bubble-diffusion-preventing treatment was performed with the cathode partition chamber under treatment conditions (temperature, voltage, time, and inrush current) shown in Table 2. The resultant aluminum piece was immediately washed with water and dried to provide an aluminum piece (test piece) after the electropolishing of each of Examples 4 and 5.

The test pieces of Examples 4 and 5 thus obtained were examined and evaluated for a bubble adhesion-suppressing property during treatment, mirror luster, point-like defects, and presence or absence of interference color in the same manner as in Examples 1 to 3 described above.

The results are shown in Table 2 together with those of Comparative Example 1 described above without the bubble-diffusion-preventing treatment.

TABLE 2 Example Comparative 4 5 Example 1 Electropolishing Presence or absence of Present Present Absent treatment bubble-diffusion-preventing treatment (cathode partition chamber) Electrolytic treatment Sulfuric Sulfuric Sulfuric solution acid-phosphoric acid-phosphoric acid-phosphoric (mass ratio) acid acid acid (1:5) (1:5) (1:5) Temperature (° C.) 60 60 60 Voltage (V) 30 30 30 Time (min) 5 5 5 Inrush current (A/dm²) 30 30 30 Bubble adhesion-suppressing Δ Δ x property during treatment Suppression effect on bubble generation ∘ ∘ x Outer appearance Point-like defect 1 1 7 observation (piece(s)/cm²) evaluation Mirror luster ∘ ∘ Δ

Examples 6 and 7 and Comparative Examples 2 and 3

After the electropolishing treatment was performed in the same manner as in Comparative Example 1 described above, film peeling treatment involving immersing the aluminum piece in a post-treatment solution under conditions shown in Table 3, to thereby peel the electropolishing film, was performed as post-treatment through use of a post-treatment solution shown in Table 3 to provide an aluminum piece (test piece) after the electropolishing of each of Examples 6 and 7 and Comparative Examples 2 and 3.

The test pieces of Examples 6 and 7 and Comparative Examples 2 and 3 thus obtained were examined and evaluated for a bubble adhesion-suppressing property during treatment, mirror luster, point-like defects, and presence or absence of interference color in the same manner as in Examples 1 to 3 described above.

The results are shown in Table 3.

TABLE 3 Example Comparative Example 6 7 2 3 Electropolishing Electrolytic Sulfuric Sulfuric Sulfuric Sulfuric treatment treatment acid-phosphoric acid-phosphoric acid-phosphoric acid-phosphoric solution acid acid acid acid (mass ratio) (1:5) (1:5) (1:5) (1:5) Temperature 60 60 60 60 (° C.) Voltage (V) 30 30 30 30 Time (min)  5  5  5  5 Inrush current 30 30 30 30 (A/dm²) Post-treatment Post-treatment 3 mass % Sulfuric NaOH Dry blasting (film peeling solution amidosulfuric acid + triethanol etching treatment) acid (pH = 0.9) amine (pH = 1.7) (pH = 13.0) Temperature 70 60 20 Room (° C.) temperature Time (min) 10  7  5 3 min Bubble adhesion-suppressing x x x x property during treatment Outer appearance Point-like  5  5 Large Large number observation defect number evaluation (piece(s)/cm²) Mirror luster Δ Δ x x

Examples 8 to 11

The pretreatment (anodic oxidation treatment) of Example 1, the bubble-diffusion-preventing treatment of Example 4, and/or the post-treatment (film peeling treatment) of Example 6 were performed as shown in Table 4 through use of the same aluminum piece having an Al purity of 99.99 mass % as that used in each of Examples 1 to 3 described above as the aluminum material, to thereby provide an aluminum piece (test piece) after the electropolishing of each of Examples 8 to 11.

The test pieces of Examples 8 to 11 thus obtained were examined and evaluated for a bubble adhesion-suppressing property during treatment, mirror luster, point-like defects, and presence or absence of interference color in the same manner as in Examples 1 to 3 described above. Further, total evaluation was performed by defining, in those evaluation items, the case where all the items were satisfactory (test piece had mirror luster in which no interference color was observed and no defects were recognized over a wide area) to be ⊚, and the case where any two or three items were satisfactory to be ◯.

The results are shown in Table 4.

TABLE 4 Example 8 9 10 11 Pretreatment Pretreatment 15 mass % 15 mass % 15 mass % — (anodic oxidation solution sulfuric acid sulfuric acid sulfuric acid treatment) Temperature (° C.) 20 20 20 — Voltage (V) 15 15 15 — Electric charge   0.1   0.1   0.1 — (C/cm²) Electropolishing Presence or Present Present Absent Present treatment absence of cathode partition chamber Electrolytic Sulfuric Sulfuric Sulfuric Sulfuric treatment solution acid-phosphoric acid-phosphoric acid-phosphoric acid-phosphoric (mass ratio) acid acid acid acid (1:5) (1:5) (1:5) (1:5) Temperature (° C.) 60 60 60 60 Voltage (V) 30 30 30 30 Time (min)  5  5  5  5 Inrush current 20 20 20 30 (A/dm²) Post-treatment Post-treatment 3 mass % — 3 mass % 3 mass % (film peeling solution amidosulfuric amidosulfuric amidosulfuric treatment) acid acid acid (pH = 0.9) (pH = 0.9) (pH = 0.9) Temperature (° C.) 70 — 70 70 Time (min) 10 — 10 10 Bubble adhesion-suppressing property ◯ ◯ X ◯ during treatment Suppression effect on bubble generation ◯ ◯ ◯ X Outer appearance Point-like defect ◯ ◯ Δ Δ observation (piece/cm²) evaluation Mirror luster ◯ ◯ ◯ ◯ Interference color ◯ X ◯ ◯ Total evaluation ⊚ ◯ ◯ ◯

REFERENCE SIGN LIST

a . . . anodic oxidation treatment step, b . . . electropolishing treatment step, S1 . . . time of immersion under no application of current in electropolishing treatment, S2 . . . time of electrolytic treatment in electropolishing treatment, S3 . . . time of completion of application of current in electropolishing treatment, 1 . . . electrolytic treatment tank, 2 . . . pretreatment tank, 3 . . . pretreatment solution, 4 . . . aluminum material (material to be treated), 5, 8 . . . cathode, 6 . . . anodic oxide film, 7 . . . electrolytic treatment solution, 9 . . . electropolishing film, 10 . . . bubbles of hydrogen gas, 11 . . . cathode partition chamber, 12 . . . discharge port. 

1. A method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material, the method comprising, as pretreatment for the electropolishing treatment, performing anodic oxidation treatment including subjecting the aluminum material to anodic oxidation in a pretreatment tank arranged separately from the electrolytic treatment tank at a voltage lower than the electrolysis voltage during the electropolishing treatment through use of a pretreatment solution formed of a polybasic acid aqueous solution having a weak ability to dissolve the aluminum material as compared to the electrolytic treatment solution, to thereby form an anodic oxide film on the surface of the aluminum material.
 2. A method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material, the electrolytic treatment tank comprising a cathode partition chamber which is formed of a material non-permeable to bubbles of hydrogen gas to be generated on a cathode serving as a counter electrode of the aluminum material and permeable to a liquid, is configured to partition the cathode from the aluminum material, and includes a discharge port for discharging the hydrogen gas out of the electrolytic treatment tank, the method comprising, during the electropolishing treatment of the aluminum material, performing bubble-diffusion-preventing treatment including: preventing the bubbles of the hydrogen gas generated on the cathode in the cathode partition chamber from diffusing to an aluminum material side; and discharging the hydrogen gas out of the electrolytic treatment tank through the discharge port.
 3. A method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material, the method comprising, as post-treatment for the electropolishing treatment, performing film peeling treatment including immersing the aluminum material after the electropolishing treatment in a post-treatment solution formed of a sulfuric acid solution having a pH of 2 or less and containing sulfuric acid and an amine, to thereby dissolve and remove an electropolishing film present on the surface of the aluminum material.
 4. A method of performing electropolishing treatment on an aluminum material including immersing an aluminum material formed of aluminum or an aluminum alloy in an electrolytic treatment solution in an electrolytic treatment tank, and applying an electrolysis voltage through use of the aluminum material as an anode, to thereby perform electropolishing treatment on a surface of the aluminum material, the method comprising performing two or three treatments selected from the group consisting of i) an anodic oxidation treatment as a pretreatment, ii) a bubble-diffusion-preventing treatment during the electropolishing treatment, and iii) a film peeling treatment of an electropolishing film as a post-treatment, wherein i) the anodic oxidation treatment includes subjecting the aluminum material to anodic oxidation in a pretreatment tank arranged separately from the electrolytic treatment tank at a voltage lower than the electrolysis voltage during the electropolishing treatment through use of a pretreatment solution formed of a polybasic acid aqueous solution having a weak ability to dissolve the aluminum material as compared to the electrolytic treatment solution, to thereby form an anodic oxide film on the surface of the aluminum material; ii) the electrolytic treatment tank comprising a cathode partition chamber which is formed of a material non-permeable to bubbles of hydrogen gas to be generated on a cathode serving as a counter electrode of the aluminum material and permeable to a liquid, is configured to partition the cathode from the aluminum material, and includes a discharge port for discharging the hydrogen gas out of the electrolytic treatment tank, and the bubble-diffusion-preventing treatment includes preventing the bubbles of the hydrogen gas generated on the cathode in the cathode partition chamber from diffusing to an aluminum material side, and discharging the hydrogen gas out of the electrolytic treatment tank through the discharge port; and iii) the film peeling treatment includes immersing the aluminum material after the electropolishing treatment in a post-treatment solution formed of a sulfuric acid solution having a pH of 2 or less and containing sulfuric acid and an amine, to thereby dissolve and remove the electropolishing film present on the surface of the aluminum material.
 5. A method of performing electropolishing treatment on an aluminum material according to claim 3 or 4, wherein the post-treatment solution comprises a sulfuric acid solution obtained by dissolving a sulfate of an amine in water, a sulfuric acid solution obtained by dissolving sulfuric acid and an amine in water, a sulfuric acid solution obtained by dissolving sulfuric acid, an amine, and a sulfate of an amine in water, or a sulfuric acid solution obtained by dissolving amidosulfuric acid or an ammonium salt thereof in water.
 6. A method of performing electropolishing treatment on an aluminum material according to claim 5, wherein the sulfuric acid solution to be used as the post-treatment solution comprises an aqueous solution of amidosulfuric acid. 